1. Field of the Invention
The present invention relates to an improved process for the preparation of chloroacetic acids, and, more especially, to an improved process for the chlorination of acetic acid wherein the byproduct crude gaseous hydrochloric acid, containing chlorine and acetyl chloride values, is purified and pure HCl recovered, while other products are recycled to the medium of chlorination.
2. Description of the Prior Art
Hydrochloric acid is produced, for example as a byproduct during the synthesis of chloroacetic acids, by reacting chlorine with acetic acid in the presence of acetic anhydride or of acetyl chloride. Such synthesis is described in Ullman's Encyclopaedia of Industrial Chemistry, 5th Edition, Vol. A6, pages 537-541.
Acetic anhydride, acetyl chloride, or mixture thereof, can be employed as a catalyst in this reaction. The reaction mechanisms are as follows: ##STR1## EQU CH.sub.3 COCl+Cl.sub.2 .fwdarw.ClCH.sub.2 COCl+HCl EQU ClCH.sub.2 COCl+CH.sub.3 COOH.fwdarw.ClCH.sub.2 COOH+CH.sub.3COCl
A fraction of the acetic anhydride is also converted to acetyl chloride: EQU (CH.sub.3 CO).sub.2 O+HCl.fwdarw.CH.sub.3 COCl+CH.sub.3 COOH
Thus produced are (i) a liquid phase containing acetic acid, monochloroacetic acid, acetyl chloride, chloroacetyl chloride, optionally acetic anhydride and the chloride thereof, and a minor amount of dichloroacetic acid and (ii) a gas phase essentially comprising hydrochloric acid and a few percent of chlorine, acetyl chloride, chloroacetyl chloride, acetic acid and monochloroacetic acid. This hydrochloric acid must be purified before being used, for example, as a reagent in an oxychlorination reaction. It is also preferable to recover the above-indicated compounds contained in the gaseous hydrochloric acid stream to permit recycling same to the synthesis of chloroacetic acids.
Techniques for the purification of such crude gaseous hydrochloric acid stream are known to this art.
FR-2,311,749 and JP-63/50,303 (Kokai) relate that purification by condensation requires low temperatures taking account of the boiling point of acetyl chloride. It is necessary to employ both pressure and a low temperature, but the impure gaseous hydrochloric acid thus provided is very corrosive to compressors.
FR-2,311,749 describes washing such crude gaseous hydrochloric acid stream with a concentrated solution of sulfuric acid and of acetic acid and then drying the washed product with concentrated sulfuric acid.
Said JP-63/50,303 (Kokai), published Mar. 3, 1988, describes washing the crude gaseous hydrochloric acid stream, containing acetyl chloride below a specified amount, with sulfuric acid. The acetyl chloride contained in the crude hydrochloric acid stream is converted into acetyl sulfate.
Nonetheless, such processes require sulfuric acid and complicated mechanical apparatus.